My engine has been running fine until 2 weeks ago. Now, it has a "speed" related engine miss. When tied to the dock under load (full throttle, low RPM's) it appears to run very steady, with very few misses. But as it winds up, it start missing. It will run great for 10 seconds, then sputter and miss, and then run a few more seconds, then sputter and miss, etc. It the brief periods it hits, it runs just great. And then it stumbles. I can't say it is totally speed related, but the problem is most pronounced as the engine winds up.<br /><br />Just FYI, the only things that have ever been done to the powerhead is plugs, fuel pump, carb rebuilding, and 1 head gasket. It is running the jets for the 75HP and timing was retarded 1 degree (just like the 75HP). I always use the engine conditioner and the anti carbon additive. And until 2 weeks ago, it was a smooth beast - incredible for a 26 year old engine!<br /><br />To date, everything I have tried hasn't worked.<br /><br />To try to resolve this condition <br /><br />1. I've used new fuel in a new tank.<br />2. Installed new plugs (the old plugs looked fine).<br />3. I've cleaned the carbs (they were rebuilt last year).<br />4. Cleaned all connections in both the powerpack and electrical bus (I "oxguarded" the clean <br />terminals)<br />5. Checked the timing and made sure it advanced properly<br />6. Checked to see that each spark was 1/2" long with a spark tester.<br />7. Checked compression (on 3 rotations, the pressure is 90PSI in each cylinder, 5 to 6 rotations, and it is 115, 120, 115 PSI.<br />8. Vacuumed checked the fuel pump to make sure it wasn't perfed and sucking gas into cylinder 3. The fuel pump vacuum line was also dry.<br />9. Pump the bulb during testing, just in case it was the fuel pump, to no effect.<br /><br />By pulling various spark plug wires at the dock, I think the problem is in Cylinder 3. When 1 & 2 are running, the engine runs very smooth. When 3 and either 1 or 2 are running, the engine will shutter slightly. However, I did run the engine with just 3 plugged in and it seemed to run OK. <br /><br />I swapped the bottom and top carb and re-synchronized. I didn't do the spark plug wire pulling again (ran out of time), but I did run the engine and the symptions were the same. <br /><br />If there is some cheap test equipment out there that will lead to a definative answer, I will buy it.<br /><br />At this point, I don't think it is ignition related, but I don't have the test equipment specified in the manual. I can't be sure. <br /><br />The cooling system on the engine is a bit weird as it doesn't have a pee hole, but the engine isn't running hot. A thermocouple had it running at 110 degrees above the inspection port. It might be running a bit cool.<br /><br />From the forum, I suspect that it could be the lower crank case seal. Is there a way to test this before pulling the engine apart?<br /><br />Is there a way to test a reed block before pulling it apart?<br /><br />I still plan to do a little more work before giving up and taking to the shop. Any suggestions?
